Nowadays, flat lighting devices gradually employ light emitting diodes (LEDs) as the light sources to replace traditional incandescent or fluorescent light tubes. When the LEDs are employed as the light sources, they usually are directly disposed on the bottom area of the flat lighting device. The LED light sources would produce Lambertian lights, and the light intensity of the lights would be strongest at 90 degree, i.e. the vertical direction to the LEDs. So, the light output from the flat lighting device may be non-uniform, and local darkness, or local brightness, i.e. the weakest or strongest light intensity areas, will be observed.
To eliminate the local darkness or local brightness, a light diffusion structure is disposed in front of the LEDs with a certain distance therebetween. However, this kind of configuration will increase the overall thickness of the flat lighting device.
Another way of reducing the local darkness or local brightness is to increase the number of LED light source. However, this kind of configuration will result in the increase of the flat lighting device overall cost. Therefore, there remains a need for an improved lighting device.